The efficiency of an underground mine and the safety of mine personnel are dependent upon the operation of the hoist. Therefore, very high standards exit for the design, construction and operation of mine hoists.
In the case of a single drum mine hoist with one conveyance used in very deep shafts in the order of 7000 ft, an electrical drive system is used for controlling the speed and a mechanical braking system is used for stopping the hoist in an emergency situation or for holding the hoist in stationary position after finishing a hoisting cycle. The stopping by the mechanical braking system in an emergency situation, referred to as an emergency stop, is initiated automatically in a case of drive failure or when a protective system detects abnormal conditions. An emergency stop can also be initiated manually by an operator of the hoist. Generally, the electrical motor must be disconnected during emergency braking.
Application of mechanical brakes during emergency stop results in deceleration of the hoist. For safety reasons, the deceleration during emergency stop must not be too small or too large. A too small deceleration results in long distances traveled before stopping, which in some cases can lead to the conveyance crashing into a shaft end. A too high deceleration subjects the people in the conveyance to excessive dynamic forces.
Due to the fact that the conveyance has a mass and is suspended on a rope, which has certain flexibility, the deceleration thereof during emergency braking results in conveyance oscillations or otherwise called bouncing. These oscillations are generated by dynamic forces developed due to speed change during emergency stop. The presence of these oscillations is undesirable as the oscillations increase the forces that the people in the conveyance are subjected to and also increase the stress in the hoist rope thereby reducing its lifetime.
Thus, the development of ways to reduce the conveyance oscillations during the hoisting cycle, and particularly during emergency stop, so as to comply with safety regulations has become imperative. A presently known controlled emergency braking method is used to provide appropriate deceleration forces so as to reduce the amplitude of the conveyance oscillations in mine hoist systems. In this method, the braking system operates with speed feedback and regulates the brake force in order to obtain proper deceleration.
An example of this controlled emergency braking method is shown in the graph of FIG. 1. The brake force is identified by curve B. The speed of hoist drum is identified by curve S, and the rope tension above the conveyance is identified by curve T. From FIG. 1, it can be seen that in the initial deceleration phase during emergency stop of a single drum mine hoist system moving in the down direction, the brake force B is increased gradually before the desired deceleration is obtained and then, in the final stage is reduced gradually. Such a control method creates an S-shaped speed curve S with the rope tension T exhibiting gradual tension changes. Notably, if the speed curve S was not S-shaped, but had a drastic acceleration/deceleration change, then the rope tension changes would not be gradual but rather step like. Hence, such rapid rope tension T changes would result in much more pronounced conveyance oscillations.
Now referring to FIG. 2, a graphical representation of emergency braking of a single drum mine hoist moving in the up direction of a shaft is shown. When moving upwards, the force of gravity plays a major part in slowing down a conveyance. In order to avoid an excessive deceleration value, the brake force B applied by the braking system must be very small. Consequently, an insignificant brake force B does not have any practical influence on the speed S of the hoist drum. The speed S curve shape is determined by the gravity force and inertia of the system. Since gravity itself creates the major downward force, the speed curve S does not have an S-shape but rather undergoes drastic deceleration changes. This can clear be seen in FIG. 2 at time 0 secs. when the electrical motor powering the hoist drum is suddenly stopped and at time 4.4 secs. when the hoist drum comes to a full stop rapidly.
Therefore, unlike in the example of FIG. 1, where the speed curve S demonstrates gradual changes, in the present case at the moment the hoist drum stops, there is a rapid change of deceleration from a value determined by the gravity force to zero. This results in rapid change in rope tension T thereby creating excessive dynamic forces.
Notably, FIG. 2 has been simplified to facilitate understanding. In reality, the change in rope tension T is not a step function as shown at 0 secs., largely due to the flexibility of the rope, but has a very fast rate of change which is much faster than in the case where the speed curve S undergoes gradual changes. Rapid, significant change of rope tension generates significant, undesirable conveyance oscillations. The significant rope oscillations caused when the hoist drum stops are clearly illustrated from 4.3 secs. onwards in FIG. 2.
Therefore, it can be seen that during emergency stop of a single drum hoist system moving in the up direction, the resulting conveyance oscillations are pronounced. There exists a need for a method of braking during emergency stop that reduces conveyance oscillations generated.